Photonic chip with edge coupler and method of manufacture

ABSTRACT

A photonic chip is disclosed that comprises a cladding material and an edge coupler. The edge coupler comprises a composite guiding structure that comprises a plurality of substantially parallel planar layers of optical guiding material. Each layer of the composite guiding structure extends into the cladding material, wherein each layer is aligned at a first edge of the photonic chip. The layers overlap along a first axis which is perpendicular to a plane of the planar layers of optical guiding material. The photonic chip is arranged for deposition of a waveguide on the cladding material, the waveguide being arranged to at least partially overlap along the first axis with a layer of the composite guiding structure. 
     Also disclosed is a method of manufacturing a photonic chip.

TECHNICAL FIELD

The present disclosure relates to a photonic chip with an edge couplerfor coupling light to an optical waveguide. The present disclosure alsorelates to a method of manufacturing a photonic chip with an edgecoupler.

BACKGROUND

Silicon photonic chips guide light through high and/or medium contrastindex waveguides. Waveguides are generally formed of a core of higherrefractive index material surround by a cladding fabricated from a lowerrefractive index material. A waveguide with a difference in refractiveindex between the core and the cladding which is less than 5% is definedas working within a low contrast regime. A difference in refractiveindex of between 5%-40% is defined as working within a medium contrastregime, and a difference of greater than 40% is defined as a highcontrast index regime.

The level of confinement of light within the waveguide is determined bythe index contrast regime. The higher the index contrast regime, themore the light is confined. A higher confinement of light leads tosmaller size and footprint of the photonic component. The mode size fora silicon waveguide on an insulator platform is less than 1 μm. However,the smaller size comes with a drawback that higher contrast indexwaveguides have increased losses, which are typically 1 dB/cm forphotonic chips formed from silicon insulator platforms.

Low contrast index waveguides include standard optical fibres (orfibers) with a core made of glass e.g. Corning (RTM). Such fibres arecommonly used in telecommunication networks to send data via themodulation of a light carrier wave over long distances of typicallyseveral kilometres. The low losses of these fibres, of the order of 0.2dB/km, make them desirable for such applications. The lower lightconfinement of these fibres gives the fibres a circular mode shape witha typical diameter size of about 10 μm.

Photonic chips may be employed to perform several operations on lightcarrier waves, including modulating, switching, routing and filtering,as well as more complex functions.

To perform any such operations, light from an incoming optical fibreshould be connected to the photonic chip. Similarly, once the operationis performed, light from the photonic chip is carried out of the chipthrough the connection of the chip to an outgoing optical fibre. Inorder to connect an incoming/outgoing fibre to a photonic chip e.g. atan interface with a photonic transceiver, the larger fibre mode (˜10 μm)should be adapted to the smaller waveguide mode (˜1 μm).

Many techniques exist to connect an incoming/outgoing optical fibre to aphotonic chip. However, they can be broadly grouped into two categories:grating couplers and edge couplers.

Grating couplers are formed in a pattern on a surface of a photonic chipe.g. a top or bottom surface. This pattern is then interfaced with anend surface of an optical fibre such that the fibre axis forms an angleof less than 10° with the chip surface normal. To attach the fibres tothe photonic chip with a grating coupler, the fibres are generallygrouped into a ribbon and attached to the grating coupler with a glassconnector e.g. a glass block. Glue or other fillers may also be used tokeep the connector in place.

Edge couplers, which are also commonly named butt couplers, performlight coupling at the edge of the photonic chip. The fibre is interfacedwith a side face of the photonic chip, where conversion between the modeof the optical fibre and the photonic chip is performed. Commontechniques to perform the mode conversion include: i) tapering of thewaveguide in combination with tapering of the fibre itself, ii)refractive index adaptation methods, such as subwavelength gratings andiii) evanescent coupling methods such as spot size converters andcombinations thereof.

Grating couplers are advantageous in certain applications owing to theirrelatively small footprint, the fewer lithography steps used in theirfabrication, and their potential for polarization diversity through theuse of dual polarization grating couplers. However, grating couplersalso suffer from a number of drawbacks.

One such drawback is that grating couplers are not broadband. Gratingcouplers can generally only accommodate incoming light that lies withina relatively narrow range, of the order of about 20 nm, around thenominal value of operation of the grating. This nominal valuecorresponds to the wavelength of maximum coupling and is determined bydesign. Grating couplers also use a vertical connector to connect to asurface of the photonic chip e.g. top or bottom surface. The verticalconnector, such as a glass block, generally carries a large horizontaland vertical footprint, of the order of mm, which is undesirable in mostapplications. Furthermore, whilst gratings with an insertion loss of theorder of 4-5 dB have a relatively simple fabrication process, producinggratings with lower losses requires more complex lithography andprocessing techniques to fabricate specific grating features andsupporting layers.

Edge couplers remedy some of the disadvantages of grating couplers, inthat they are broadband and allow a flat connection of the fibres to thephotonic chip, which reduces the footprint in the vertical dimension.However, for certain applications, the edge coupler may require arelatively large horizontal footprint for the mode adaptation betweenthe optical fibre and the photonic chip. The larger horizontal footprintcan also increase the overall cost of the photonic chip.

Several methods have been investigated to provide mode adaption betweenedge couplers and optical fibres with a reduced footprint. A class ofmethods that has been widely investigated in this field aims atachieving the conversion of the mode between the fibre and the chip viamulticore waveguides, or by including layers of materials with differentrefractive indices within the edge coupler. These methods may be basedon: i) evanescent coupling of the incoming mode with a distribution ofwaveguide cores, ii) transfer of light to an output waveguide, iii)shaping the effective refractive index via layers of multiple materials,and iv) adiabatic index/shape variation of guiding elements andcombinations thereof.

U.S. Pat. No. 7,164,838 describes a method in which the adaptation isimplemented via multiple core planar waveguides, the waveguidescomprising a main core surrounded by a lower core layer and an uppercore layer. The upper core layer and the lower core layer both extendlaterally either side of the main core. The upper core layer and thelower core layer are separated from the main core by a cladding, withfurther cladding material on top of the upper core layer and underneaththe lower core layer. The cladding has a refractive index such that thedifference between the refractive index of the cladding and the upperand lower core layers is within a low index contrast regime, and thedifference between the cladding and the main core is within a high indexcontrast regime. In this method the core layers enlarge the mode of thecentral main core to provide spatial mode matching with the input lightfrom an optical fibre, for example. The light therefore graduallytransfers from the upper and lower layers to the central main core.

The method in U.S. Pat. No. 7,164,838 has disadvantages in terms offabrication. In this method, the waveguide is buried under substantiallayers of cladding material. This makes it difficult to access thewaveguide to provide electrical couplings to optical components in thesilicon layer, such as heaters for thermal tuning or drive electrodesfor modulators. Heaters are generally fabricated in the back-end of aproduction line (BEOL). The techniques used to deposit metal on a chipgenerally use physical vapor deposition or sputtering-etching processes,which often involves the contamination with metals, which may precludecertain processing operations in the front-end of line (FEOL). Thefabrication of waveguides is generally carried out in the FEOL, whichinvolves a CMOS-compliant environment strictly restricting contaminationby some metals. A buried waveguide therefore presents a number ofundesirable challenges in the fabrication process of the chip.

One technique to access a buried waveguide is to etch away part of thecladding at a location away from the edge coupler. However, this processresults in a chip surface layer with a varying thickness of the claddinglayer. The presence of a non-uniform surface leads to limitations in thedesign freedom for the location of the heaters, and also results inincreased processing complexity and cost. For example, photo-resistdeposited on the surface of the chip may have an irregular thickness.Thinner regions of photo-resist may lead to areas of cladding beingsubstantially uncovered. Uneven or overly thick regions of photo-resistcan lead to limitations on the resolution of the features that can befabricated.

U.S. Pat. No. 9,588,298 describes a similar concept that uses multiplecores. The method uses several core layers of a semiconductor materialwith higher refractive index, interleaved with a cladding material oflower refractive index. These core layers form a composite input portand may have a geometrical arrangement that forms a 2D or linear array.The waveguide cores are medium contrast waveguides, with cores formed ofa semiconductor material such as silicon nitride, and cladding formed ofa dielectric material such as silicon dioxide. The output waveguide coreis located below the other cores when viewed along the axis ofpropagation of light along the waveguide. The output waveguide core istapered such that the end of the core nearest the edge of the chip islarger in dimension than the end of the waveguide located further intothe chip. This tapering structure can aid in the coupling of light fromthe edge coupler to the photonic chip. With this edge couplerconfiguration, incoming light is coupled into the multiple core layersabove the output waveguide core, and light is evanescently coupled fromthese cores to the output waveguide core below them.

The method described in U.S. Pat. No. 9,588,298 involves severalfabrication steps to superpose the input core layers on top of theoutput waveguide core. This method also suffers from the limitationsrelating to the coupling of components such as heaters to the outputwaveguide and the contamination of metals in BEOL, which limits theprocessing in the FEOL. Furthermore, this method involves a largernumber of layers on top of the main core, which puts the main core underincreased stress. In this method, the output waveguide needs to becarefully aligned with the other layers, with the output waveguidesuffering the same increased stress level as the other cores. This leadsto very little tolerance in the alignment of the waveguide, andconsequent difficulties in manufacture.

Furthermore, this method uses prongs rather than layers, which are moredifficult to manufacture.

The issue of introducing metal routes in a multi layered edge coupler isaddressed in US patent application no. 2016/0266321 and in U.S. Pat. No.9,933,566. The proposed methods involve the creation of multiple layersof metals connected by vias. These methods have the drawback ofinvolving more fabrication stages, which increases the probability ofoccurrence of fabrication defects.

In general prior art edge couplers further suffer from issues withtesting the optical components during the fabrication process. It is notpossible to include test structures with vertical output (e.g. gratingcouplers) to assess the quality of fabrication of waveguides beforemetal deposition or before dicing. The inability to perform testingduring the production process can significantly reduce the efficiencyand yield of the production process.

Edge coupler solutions have been proposed to overcome this limitation byplacing the output waveguide on top of the multicore structure, forexample, as described in U.S. Pat. No. 9,274,275. In this fabricationmethod, the device layer (i.e. the layer containing the output waveguideand main photonic chip elements) is transferred on top of the multi-core stack by means of a mechanical transfer (e.g. flip-chip or waferbonding). Such mechanical transfers are necessary because thermalprocesses (e.g. high-temperature annealing) may have to be carried outseparately on the multi-core stack and on the device layer. The method,however, has the drawback of posing significant design limitations tothe type and the geometry of the device layer. The technique is feasiblein silicon oxide substrates, but cannot be extended to materials such assilicon nitride, where the device layers are generally affected bysignificant stress and may not be readily transferrable. Also,non-planar devices with, e.g., metal plugs for resistive heaters orother similar protruding structures, may pose additional challenges to awafer-scale, transfer-based approach.

Additionally, the edge couplers presented in the prior art do notaccount for the possibility to manage polarization diversity of light.

SUMMARY

It is an aim of the present disclosure to provide an apparatus andmethod of manufacture thereof which obviates or reduces at least one ormore of the disadvantages mentioned above.

According to a first aspect of the present disclosure there is provideda photonic chip that comprises a cladding material and an edge coupler.The edge coupler comprises a composite guiding structure that comprisesa plurality of substantially parallel planar layers of optical guidingmaterial. Each layer of the composite guiding structure extends into thecladding material, wherein each layer is aligned at a first edge of thephotonic chip. The layers overlap along a first axis which isperpendicular to a plane of the planar layers of optical guidingmaterial. The photonic chip is arranged for deposition of a waveguide onthe cladding material, the waveguide being arranged to at leastpartially overlap along the first axis with a layer of the compositeguiding structure.

According to examples of the present disclosure the photonic chip may bearranged for deposition of a waveguide on an upper surface of thecladding material.

According to examples of the present disclosure a width of each layer ofthe composite guiding structure may decrease from a first end of thelayer, which is at the first edge of the photonic chip, to a second endof the layer opposite the first end.

According to examples of the present disclosure a length of at least oneof the layers of the composite guiding structure may be different to alength of at least one other of the layers of the composite guidingstructure.

According to examples of the present disclosure a length of each layerof the composite guiding structure is different to a length of each ofthe other layers of the composite guiding structure. According to suchexamples, the length of each of the plurality of layers may increasewith reduced distance from a surface on which the photonic chip isarranged for deposition of the waveguide.

According to examples of the present disclosure the photonic chip may bearranged for deposition of a waveguide on a surface of the claddingmaterial, where waveguide may be arranged to at least partially overlapa layer of the composite guiding structure that is closest to thewaveguide. According to such examples the waveguide may be arranged toat least partially overlap with only the layer of the composite guidingstructure that is closest to the waveguide.

According to examples of the present disclosure the composite guidingstructure may comprise three substantially parallel planar layers ofoptical guiding material.

According to examples of the present disclosure the composite guidingstructure may comprise an upper layer, a middle layer, and a lowerlayer, and wherein a length of the middle layer is greater than twice alength of the lower layer.

According to examples of the present disclosure the composite guidingstructure may comprise an upper layer, a middle layer, and a lowerlayer, and wherein a length of the upper layer is greater than twice alength of the middle layer.

According to examples of the present disclosure the cladding materialmay comprise silicon dioxide.

According to examples of the present disclosure a difference between arefractive index of the layers of optical guiding material and arefractive index of the cladding material may be within a mediumcontrast index regime.

According to examples of the present disclosure the optical guidingmaterial comprises silicon nitride.

According to examples of the present disclosure a separation along thefirst axis between the each of the plurality of layers in the compositeguiding structure may be equal. According to such examples a separationalong the first axis between the composite guiding structure and asurface of the cladding material for deposition of a waveguide thereon,may be greater than the separation between each of the plurality of thelayers.

According to examples of the present disclosure the photonic chipfurther comprises a marker element, the marker element may indicate aposition of the edge coupler.

According to examples of the present disclosure the photonic chip mayfurther comprise a waveguide deposited on the surface of the claddingmaterial, the waveguide comprising features as set out in any one of thepreceding claims.

According to examples of the present disclosure the waveguide maycomprise a first end that at least partially overlaps with a layer ofthe composite guiding structure in a direction of propagation of lightalong a layer of the plurality of layers, and a width of the waveguidemay increase away from the first end.

According to a second aspect of the present disclosure there is provideda method of manufacturing a photonic chip. The method comprises:providing cladding material; forming an edge coupler, comprisingdepositing a composite guiding structure comprising a plurality ofsubstantially parallel planar layers of optical guiding material;wherein each layer of the composite guiding structure extends into thecladding material, wherein each layer is aligned at a first edge of thephotonic chip and wherein the layers overlap along a first axis which isperpendicular to a plane of the planar layers of optical guidingmaterial; and forming a surface of the photonic chip for deposition of awaveguide on the cladding material, the waveguide being arranged to atleast partially overlap along the first axis with a layer of thecomposite guiding structure.

According to examples of the present disclosure forming the edge couplermay comprise (a) removing a portion of the cladding material at a firstend of the cladding material along a first axis to form a claddingmaterial surface; (b) fabricating a planar layer of optical guidingmaterial on the cladding material surface; (c) depositing a claddingmaterial onto the layer of optical guiding material to form a newcladding material surface; and (d) repeating steps (b) and (c) to form acomposite guiding structure comprising a plurality of substantiallyparallel planar layers of optical guiding material interleaved withcladding material.

According to examples of the present disclosure step (a) may compriseperforming a stress de-compensation procedure and step (c) may compriseperforming a stress compensation procedure

According to examples of the present disclosure step (b) may comprisedepositing optical guiding material and removing optical guidingmaterial to fabricate the planar layer of optical guiding material.

According to examples of the present disclosure the method may furthercomprise fabricating a marker element indicating a position of thecomposite guiding structure.

According to such examples the marker element may be located on anexterior of the cladding material or may be buried in the claddingmaterial.

According to examples of the present disclosure the method may furthercomprise fabricating a waveguide on the photonic chip. According to suchexamples, fabricating the waveguide may comprise aligning the waveguidebased on the marker element.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention, and to show moreclearly how it may be carried into effect, reference will now be made,by way of example, to the following drawings, in which:

FIG. 1 is a block diagram illustrating functional units in a photonicchip;

FIGS. 2 a-2 c are diagrams illustrating a photonic chip incross-sectional and perspective view;

FIG. 3 is diagram illustrating the intensity of light propagating in aphotonic chip;

FIG. 4 is a diagram illustrating a mode field distribution of a siliconwaveguide;

FIG. 5 is a diagram illustrating a mode field distribution of a siliconnitride waveguide;

FIG. 6 is a diagram illustrating a mode field distribution of a photonicchip;

FIG. 7 is a flow chart illustrating process steps in a method ofmanufacturing a photonic chip; and

FIG. 8 is another flow chart illustrating process steps in a method ofmanufacturing a photonic chip.

DETAILED DESCRIPTION

Aspects of the present disclosure provide a photonic chip that comprisesan edge coupler for coupling light between the photonic chip and anoptical fibre. The edge coupler comprises a composite guiding structurethat enables mode adaptation between an optical fibre and an opticalwaveguide, where the waveguide may be comprised in the photonic chip.The waveguide may be located above the composite guiding structure, oncethe composite guiding structure has been formed. The edge couplerstructure gives the photonic chip a relatively small horizontal andvertical footprint. Aspects of the present disclosure also provide amethod of manufacturing of a photonic chip that comprises an edgecoupler for coupling light between the photonic chip and optical fibre.The method of manufacture, and the structure of the photonic chip, allowfor a greater design freedom of other structures which may be comprisedon the photonic chip following its fabrication.

The method of manufacture and the structure of the photonic chip furtherallow for testing of the edge coupler structure throughout itsfabrication, which increases production yield and efficiency.

FIG. 1 is a block diagram illustrating functional units in a photonicchip 100. Photonic chip 100 comprises a cladding material 110 and anedge coupler 120, which comprises a composite guiding structure 122. Thecomposite guiding structure 122 comprises a plurality of substantiallyparallel planar layers of optical guiding material. In some examples,planar may mean that the layers have a substantially two-dimensionalcharacteristic, i.e. that they extend principally along two dimensionsforming a plane. In some examples, planar may mean that the thickness ofthe layer is smaller than its width and length by at least two orders ofmagnitude. In some examples, parallel may mean that the layers eachextend in a direction along an X axis (shown in FIG. 2 a ), such thatthe layers would never intersect along the X axis. In some examples,planar may mean that a planar surface of each layer extends across an XYplane.

Each layer of the composite guiding structure 122 extends into thecladding material where each layer is aligned at a first edge of thephotonic chip. In some examples, the layers being aligned may mean thatthe layers are aligned along a Z axis, perpendicular to the XY plane. Insome examples, aligned may mean that one edge of the layers are aligned,e.g. extend to the same point on the X axis. In some examples, alignedmay mean that the layers are arranged along an axis such that they areat least partially superimposed on one another. In some examples,aligned may mean that the layers are arranged along a first plane thatis perpendicular to a plane of the planar layers such that an edge ofthe planar layers lies on the first plane. The layers are arranged tooverlap along a first axis which is perpendicular to a plane of theplanar layers of optical guiding material. In some examples, the firstaxis may therefore comprise the Z axis. In some examples, a plane of theplanar layers may comprise a planar surface of the layers. In someexamples, each planar surface of each layer may therefore be arranged toalign and overlap with the other planar layers along the first axis. Insome examples, the first axis may comprise a depth of the chip.

As will be described in more detail below, the structure of thecomposite guiding structure 122 provides efficient mode adaption betweena waveguide and an incoming/outgoing optical fibre.

The photonic chip 100 is arranged for deposition of a waveguide on thecladding material 110. In some examples, the cladding material 110 maycomprise a surface for fabrication of a waveguide thereon. In someexamples, the surface of the cladding material may comprise an uppersurface of the cladding material 110. A waveguide formed on the claddingmaterial 110, for example on the upper surface of the cladding material110, may be arranged to at least partially overlap along the first axiswith a layer of the composite guiding structure 122. In some examples,the waveguide may be arranged to partially overlap with a layer of thecomposite guiding structure 122 along a direction in which, in use,light may propagate along one or more of the layers. In some examples,the waveguide may also be arranged to align with the layers along adirection in which, in use, light may propagate along one or more of thelayers. As will be described in more detail below, the arrangement ofthe waveguide relative to the composite guiding structure 122 enableslight to be coupled from the composite guiding structure 122 to thewaveguide and vice versa.

In some examples, the waveguide comprises a first end that is arrangedto at least partially overlap with a layer of the composite guidingstructure. In one example, the first end of the waveguide is locatednearest the first edge of the photonic chip 100. The first end of thewaveguide is spaced apart from the first edge of the photonic chip 100.In one example, the waveguide may comprise a tapered profile, such thata dimension of the waveguide increases from the first end towards asecond end of the waveguide opposite the first end. In one example, thedimension may comprise the width of the waveguide (e.g. in the Y axis).As will be described in more detail below, the tapering of the waveguidemay assist in adapting the mode of the optical fibre to that of thewaveguide and vice versa.

In some examples, each layer of the composite guiding structure may betapered along its length i.e. a dimension of each layer may decreasefrom one end of the layer to the opposite end. In one example, a widthof each layer (e.g. in the Y axis) of the composite guiding structuremay decrease from a first end of the layer, at the first edge of thechip 100, towards a second end of the layer, opposite the first end. Inother examples, one or a subset of the layers may be tapered and theother layers may not be tapered.

In some examples, a length of at least one of the layers of thecomposite guiding structure 122 may be different to a length of at leastone other of the layers of the composite guiding structure 122. In thisexample, the length may refer to the length of each layer that extendsfrom its first end, at the first edge of the chip, to the second end,opposite the first end, along an axis in which, in use, light maypropagate along one or more of the layers. This is the length in theX-axis. It will therefore be appreciated that in this example, thelength that one of the layers extends into the chip 100 may differentthan the other layers.

In some examples, the length of each layer of the composite guidingstructure 122 may be different to the length of each of the other layersof the composite guiding structure 122. The length of each layer fromits respective first end to its respective second end may therefore bedifferent for each layer. In other examples, the length of each layermay be the same.

In some examples, where a waveguide is deposited and/or fabricated onthe cladding material 110, the waveguide may be arranged to at leastpartially overlap with the layer of the composite guiding structure thatis closest to the waveguide. For example, where the waveguide isdeposited on an upper surface of the cladding material 110 of thephotonic chip 100, the waveguide may be arranged to at least partiallyoverlap a second end, furthest from the first edge of the photonic chip,of the uppermost layer of the composite guiding structure 122. The uppersurface of the cladding material may be planar, allowing for addition ofthe waveguide on the photonic chip. The planar upper surface which maybe parallel to the layers may provide for the waveguide to be parallelto the layers. The planar upper surface may extend continuously over thelayers of the composite guiding structure and may comprise an area forthe waveguide to be deposited thereon.

In some examples, the waveguide may further be configured to partiallyoverlap with only the layer of the composite guiding structure 122 thatis closest to the waveguide. For example, where the waveguide iscomprised on the upper surface of the cladding material 110, thewaveguide may partially overlap only with the uppermost layer of thecomposite guiding structure 122 and may not overlap any of the otherlayers of the composite guiding structure 122. Overlap may refer tooverlapping in the XY plane. In such examples, the uppermost layer maytherefore be different length than all other layers of the compositeguiding structure 122 such that the waveguide can be arranged topartially overlap the uppermost layer and not overlap any other layer.

In some examples, the composite guiding structure 122 may comprise threesubstantially parallel planar layers of optical guiding material.However, in other examples, the composite guiding structure may comprisefewer than or more than three layers. The composite guiding structure122 may comprise an upper layer, a middle layer and a lower layer. Thelength of the middle layer may be longer than, e.g. greater than twicethe length of, the lower layer. In another example, the length of theupper layer may be longer than, e.g. greater than twice the length of,the middle layer. In such examples, the length may comprise the distancebetween the first end of the layers, nearest the first edge of thephotonic chip 100, and the second end, opposite the first end. In suchexamples, the length which the upper, middle and lower layers extendinto the chip 100 may therefore be different.

In some examples, the plurality of substantially parallel planar layersof optical guiding material may each be separated by a layer of claddingmaterial. In other examples, the plurality of substantially parallelplanar layers may be separated by another insulating material other thanthe cladding material. In other examples, a combination of the claddingmaterial and an insulating material may be used to separate theplurality of substantially parallel planar layers.

In some examples a difference between a refractive index of the layersof optical guiding material and a refractive index of the claddingmaterial may be within a medium contrast index regime. As describedabove, a difference in refractive indices of between about 5% and about40% may correspond to materials that are within a medium refractiveindex regime. The refractive index of the optical guiding material maytherefore be greater than the refractive index of the cladding material.In one example, the cladding material may comprise silicon dioxide. Inone example, the optical guiding material may comprise silicon nitride.

In some examples, a separation along the first axis that extends along adepth of the chip 100 (e.g. in the Z-axis or perpendicular to adirection of propagation of light along the layers) may be equal i.e.the separation between each of the layers may be substantially the same.However, in other examples, the separation between each of the layersmay be different. In one example, a separation along the first axisbetween the composite guiding structure 122 and a surface of thecladding material 110 for deposition of a waveguide thereon, is greaterthan the separation between each of the plurality of the layers.Therefore in one example, the surface of the cladding material 110 maycomprise an upper surface and the distance between the upper surface andan uppermost layer of the plurality of layers may be greater than aseparation between each of the plurality of layers.

In some examples, the photonic chip 100 may further comprise a markerelement indicating a position of the edge coupler 120. As will bedescribed in more detail below, the marker element may be buried in thecladding material 110 and/or may be located on an exterior surface e.g.an upper surface of the cladding material 110. In one example, duringfabrication of the waveguide, the waveguide may be aligned with thecomposite guiding structure 122 based on the marker element.

An example implementation of a photonic chip 200 is illustrated in FIG.2 a . The photonic chip 200 illustrates one way in which the functionalblocks of the photonic chip 100 may be realised, as well as illustratingadditional elements which may provide enhanced or additionalfunctionality. Specifically, the photonic chip 100 is illustrated in astate in which it is arranged for deposition of a waveguide on thecladding material but before such deposition, whereas the photonic chip200 is illustrated following deposition of a waveguide.

Referring to FIG. 2 a , the photonic chip 200 comprises claddingmaterial 210 and edge coupler 220. Photonic chip 200 may also comprise asubstrate 280. In one example, the substrate 280 may comprise silicon.As will be described in more detail below, the substrate 280 andcladding material 210 may be comprised as part of a wafer and the edgecoupler 220 may be fabricated into the wafer to form the photonic chip200.

The edge coupler 220 comprises a composite guiding structure 222, whichcomprises a plurality of substantially parallel planar layers of opticalguiding material. The plurality of optical guiding material layers maycomprise three layers: a first layer 252, a second layer 254 and a thirdlayer 256. The X axis (length) and Z axis (depth or height) are shown.The Y axis (width) is perpendicular to these axes, i.e. into the page.

Each of the layers comprises a first end, which extends from a firstedge 240 of the photonic chip 200. As will be described below, the firstedge 240 may comprise an interface for connecting an optical fibre tothe edge coupler 220. Each layer extends into the photonic chip 200 andfurther comprises a second end, which is opposite the first end. Firstlayer 252 therefore comprises a first end 252 a and second end 252 b,second layer 254 comprises a first end 254 a and second end 254 b andthird layer 256 comprises a first end 256 a and second end 256 b. Theplurality of layers are aligned with each other along a first axis Z,which is perpendicular to a plane of the planar layers of opticalguiding material 252-256. As such, the layers have a common position ofthe first end in the X axis direction. The first axis Z also extendsalong a depth of the chip 200 and is perpendicular to an axis in which,in use light may propagate along each of the layers 252, 254, 256, alongthe X-axis.

Referring again to FIG. 2 a , the plurality of substantially parallelplanar layers are each separated by a layer of the cladding material210. The first layer of optical guiding material 252 and the secondlayer of optical guiding material 254 are separated by a first layer ofcladding material 226 a. The second layer of optical guiding material254 and the third layer of optical guiding material 256 are separated bya second layer of cladding material 226 b. Although the claddingmaterial is described as layers, cladding material may also beconsidered as homogenous or continuous.

Referring again to FIG. 2 a , the photonic chip 200 further comprises awaveguide 230 which is arranged to at least partially overlap along thefirst axis Z with at least one layer of the composite guiding structure222. The waveguide 230 comprises a first end 230 a located nearest thefirst edge 240 of the photonic chip 200 and a second end 230 b locatedopposite the first end 230 a of the waveguide 230.

As illustrated in FIG. 2 a , the waveguide 230 is arranged to overlapthe third layer 256 of the composite guiding structure 222. Thewaveguide 230 is separated from the third layer 256 by a third layer ofcladding material 228.

FIG. 2 b and FIG. 2 c show another example of the photonic chip 200illustrating how the functional blocks of the photonic chip 100 may berealised, as well as illustrating additional elements which may provideenhanced or additional functionality. Elements with correspondingreference numerals in FIG. 2 b and FIG. 2 c have the same definition asgiven in FIG. 2 a.

Referring to FIG. 2 b , first layer 252, second layer 254 and thirdlayer 256 all comprise a length that extends into the photonic chip 200from the first edge 240 that is different. In one example, the length ofthe second layer 254 is greater than twice the length of the first layer252. In another example the third layer 256 is twice as long as thesecond layer 254. In one example, the first layer 252 may be about 95 μmlong from first end 252 a to second end 252 b. In one example, thesecond layer 254 may be about 235 μm long from first end 254 a to secondend 254 b. In one example, third layer 256 may be about 635 μm long fromfirst end 256 a to second end 256 b.

Referring to FIG. 2 c , the differing lengths of the first layer 252,second layer 254 and third layer 256 is also illustrated between theirrespective first and second ends. As also illustrated in FIG. 2 c , eachof the plurality of layers 252-256 may be tapered i.e. a width of eachlayer may decrease from its respective first end to respective secondend. In one example, a width of each first end 252 a-256 a of each layer252-256 is about 10 μm and a width of each second end 252 b-256 b ofeach layer 252-256 is about 5 μm.

In some examples, at least a region of the waveguide 230 may also betapered such that a width of the waveguide 230 increases from one endtowards another end of the region.

In some examples, this may be referred to as a tapered region. Referringagain to FIG. 2 b , the tapered region of the waveguide 230 may extendfrom the first end 230 a to the second end 230 b. Therefore, in someexamples, the waveguide 230 may extend beyond the second end 230 bfurther into the chip 200, such that the second end 230 b of the taperedregion is merely a reference point indicating the location of thewaveguide at which the tapering ends. A width of the tapered region ofthe waveguide 230 at the first end 230 a may be smaller than a width ofthe tapered region of the waveguide 230 at the second end 230 b. In oneexample, the width of the first end 230 a may be about 0.4 μm and thewidth at the second end 230 b may be about 1.5 μm.

Referring again to FIG. 2 b , the waveguide 230 may be arranged tooverlap with the third layer 256. A portion of the waveguide 230 and aportion of the third layer 256 may be arranged to overlay each otherover an overlap length 260. In one example the overlap length 260 may beabout 170 μm.

Referring still to FIG. 2 b , the plurality of substantially parallelplanar layers of optical guiding material may each be separated by alayer of cladding material. In one example, a thickness of the layers ofcladding material separating the plurality of layers of optical guidingmaterial may be equal. In the illustrated example, the first layer ofcladding material 226 a and the second layer of cladding material 226 bboth have a thickness of about 2.8 μm.

In one example, the separation between the layers of optical guidingmaterial may be related to the thickness of the layers of opticalguiding material. As will be described in more detail below, a change inthe thickness of the layers of optical guiding material or theseparation between the layers may result in a change in the effectiverefraction index of the composite guiding structure. Changes in theeffective refraction index further effect the adaption of the modebetween an optical fibre and a waveguide. Therefore changes in thethickness of the layers of optical guiding material or the separationbetween the layers should be balanced such that adaptation of the modebetween the optical fibre and the waveguide may be achieved.

In one example, a thickness of each layer of optical guiding materialmay be about 50 nm. Therefore, in one example, if the thickness of asubset or all of the layers of optical guiding material 252-256 were tochange, this may result in a change in the separation between the layersof optical guiding material in order to adapt the mode between anoptical fibre and the waveguide 230. For example, if the thickness ofthe first or second layers were to change, then the thickness of thefirst layer of cladding material 226 a and the second layer of claddingmaterial 226 b may also change. However, it will also be appreciatedthat the stoichiometric composition of the optical guiding material inan edge coupler should be preserved during fabrication. Therefore, inone example the thickness of each of the layer of optical guidingmaterial 252-256 and the thickness of the layers of cladding material226 a, 226 b, separating the layers of optical guiding material shouldalso be selected such that the stoichiometric composition is preservedduring fabrication of the photonic chip 200.

A separation between the layer of optical guiding material nearest thewaveguide 230, i.e. the third layer 256, and the waveguide may begreater than the separation between each of the layers of opticalguiding material e.g. first layer of cladding material 226 a and secondlayer of cladding material 226 b. In one example, the third layer ofcladding material 227 between the third layer 256 and the waveguide 230may be about 3.1 μm.

Referring again to FIG. 2 b , an upper portion of the cladding material210, which in some examples may comprise an upper surface of thecladding material 210, and the waveguide 230 may be separated by afourth layer of cladding material 228. In one example, the fourth layer228 may comprise a thickness of about 2.5 μm. A lower portion of thecladding material 210, which in some examples may comprise a lowersurface of the cladding material 210, may be separated from the firstlayer of optical guiding material 252 by a fifth layer of claddingmaterial 229. In one example, the fifth layer of cladding material 229may separate the first layer of optical guiding material 252 fromsubstrate 280. In one example, the fifth layer of cladding material 229may comprise a thickness of about 7 μm.

The length of the planar layers (X axis) of the composite guidingstructure is larger than then width of the planar layers (Y axis). Thewidth of the layers is larger, or approximately equal to, the depth(thickness) of the layers (Z axis).

Referring again to FIG. 2 b , photonic chip 200 may interface with anoptical fibre 270 at the first edge of the chip 240. The optical fibre270 comprises fibre cladding material 272 and a fibre core 274. Inoperation, the optical fibre 270 interfaces with the chip at the firstedge 240 for the delivery of light to the photonic chip 200. The opticalfibre 270 may comprise a single-mode fibre, a multimode fibre or amulti-core fibre. The optical fibre 270 may be coupled at the interfaceof the chip 200 at the first edge 240 via a number of means, as oneskilled in the art will be familiar with. In one example, the fibre 270may be coupled to the chip via a V-shaped groove to hold the fibre 270in place at the interface at the first edge 240.

The fibre core 74 is aligned with the composite guiding structure, i.e.layers of optical guiding material 252-256. Light from (or to) theoptical fiber can enter (or exit) the composite guiding structure. Thecomposite guiding structure and fiber core 274 overlap in the YZ plane.

The light delivered from such an optical fibre 270 may be operated uponby optical modules comprised in the photonic chip 200 for example viamodulators or filters. For light to undergo such operations, the lightshould be coupled from the optical fibre 270 to the modules of thephotonic chip via the edge coupler 220.

In operation, the structure of the composite guiding structure, and thearrangement of the waveguide relative to the composite guidingstructure, enable mode adaption between an optical fibre 270 and thewaveguide 230 for coupling of light to/from the photonic chip. Thestructure of the composite guiding structure formed from the layers ofoptical guiding material, which may comprise a high refractive indexmaterial, and the cladding material, which may comprise a low refractiveindex material, results in a structure with an average refractive indexthat is capable of efficiently adapting the mode between an opticalfibre and the waveguide. In one example, the average refractive index ofthe composite guiding structure may be designed such that the fielddistribution of the mode in the composite guiding structure has asuperposition integral greater than 90% with the field of the input modefrom the optical fibre.

FIG. 3 is diagram illustrating the intensity of light propagating in aphotonic chip, in which the functional blocks of the photonic chip 100,or the elements in photonic chip 200, may be realised. Elements withsimilar reference numerals in FIG. 3 are given the same definition asgiven in FIGS. 2 a to 2 c.

The structure of the composite guiding structure enables light to becoupled evanescently from the layers of optical guiding material 352-356to the waveguide 330. Referring to FIG. 3 , light may be delivered tothe photonic chip via a light delivery means such as an optical fibre atan interface between the chip and fibre e.g. at the first edge of thephotonic chip. The increasing length of the layers 352-356 with reduceddistance to the waveguide 330 causes the field distribution of the modeto move progressively towards the waveguide 330, with increased distanceinto the chip from the first edge.

Light may be incident on the composite guiding structure from the firstedge, delivered from a means such as an optical fibre. Light thereforepropagates from left to right along the x axis, as viewed in FIG. 3 .The higher refractive index of the layers of optical guiding materialcompared to the cladding material means that light is transmitted intothe layers 352-356 and is guided along each layer from the interfacebetween the optical fibre and the chip at the first edge of the chip.The area occupied by the three layers means the majority of theintensity of the light from the optical fibre is coupled into thecomposite guiding structure.

Referring to FIG. 3 , at a distance 0 along the x axis, the intensity oflight may be substantially evenly spread across the three layers352-356. As light travels along the first layer 352, at distance 0.095mm along the x axis, the light in the first layer 352 reaches the secondend of the first layer 352, opposite the first end of the layer at thefirst edge of the chip. The separation between the first layer 352 andthe second layer 354 is designed such that light at the second end ofthe first layer 352 is evanescently coupled to the second layer 354. Atthe distance 0.095 microns along the x axis of the chip, the light inthe composite guiding structure is therefore distributed between thesecond layer 354 and the third layer 356. At 0.235 microns along the xaxis, the light in the second layer 354 has reached the second end ofthe second layer 354, opposite its first end at the first edge of thechip. The separation between the second layer 354 and the third layer356 is designed such that light from the second layer 354 isevanescently coupled to the third layer 356. At distance 0.235 mm alongthe x axis, all of the light in the composite guiding structure istherefore comprised in the third layer 356.

The planar design of the layers of composite guiding material 352-356,the appropriate separation of cladding material between the layers352-356, and the increasing length of the layer 352-356 with reduceddistance to the waveguide 330 therefore all assist in moving theintensity of light and the field distribution of the mode towards thewaveguide 330 with increased distance into the chip along the x axis.The length of the layers 352-356 is therefore designed to minimizelosses and obtain smooth mode conversion along the direction ofpropagation of the light along the layers 352-356.

The waveguide 330 is arranged to overlap with the third layer 330 by adistance of 170 μm. With this overlap, and a third layer length of 635μm, at 0.465 mm into the chip along the x axis, light from the thirdlayer 356 begins to evanescently couple to the waveguide 330. At 0.635mm along the X axis, light in the third layer 356 has reached the end ofthe third layer 356 and substantially all of the light in third layer356 may have thus been evanescently coupled into the waveguide 330. Theseparation between the third layer 356 and the waveguide 330, as well asthe overlap length between the third layer 356 and the waveguide 330, isdesigned such that the mode field distribution of the light is coupledinto the waveguide 330.

The light in the waveguide 330 is thus guided to (or from) the othermodules of the photonic chip, where the light may be operated upon. Asimilar procedure is also carried out to couple light from the waveguide330 to an optical fibre at an interface between the chip and an opticalfibre. The light is evanescently coupled from the waveguide 330 to thelayers of the composite guiding structure and finally transmitted out tothe optical fibre.

As described above, the layers 352-356 and the waveguide may eachcomprise a tapered profile e.g. a tapered width. The width of the layersmay decrease with increased distance into the chip, which helps adaptthe mode of the fibre to the mode of the waveguide 330. Conversely, thewaveguide 330 may comprise a width which increases with distance intothe chip. This tapering may also assist in coupling light from the thirdlayer 356 to the waveguide 330. The width of the waveguide then widensto guide light into the chip for subsequent operations on the light.

As described above, the separation between the layers of optical guidingmaterial 352-356 is designed to be dependent upon the thickness of thelayers of optical guiding material 352-356, as changes in either thethickness or the separation can affect the effective refractive index ofthe composite guiding structure. However, the separation is alsodesigned to provide efficient evanescent coupling between the layers352-356 and to the waveguide 330. The separation should also be chosenbased on the mode field diameter of the optical fibre, which maytypically be about 10 μm. The separation between the layers 352-356should therefore be designed to provide efficient coupling of lightbetween the edge coupler and the fibre with minimal loss of intensity atthe interface between the edge coupler and fibre.

The thickness of the guiding material layers 352-356 may be designed forefficient guiding of light along the layers 352-356. The thickness maybe designed, in combination with the separation between the layers352-356, to match the mode field profile of the fibre to provideefficient coupling between the edge coupler and the fibre. If thethickness is too high then separation of modes can occur, whereas if thethickness is too thin then edge effects can lead to undesirable opticalphenomena. The thickness of the layers 352-356 may therefore be designedto avoid any such undesirable qualities and instead to provide efficientradiation attraction and mode guiding.

The refractive index of the materials used in the composite guidingstructure and the chip should be chosen to aid in mode adaption andefficient transfer of light. Higher refractive index materials e.g. Sisubstrate that may in some examples, form the base of the chip may bekept away from the lower conversion region of the edge coupler, to avoidundesirable absorption and optical losses. Refractive indices of thecladding material and the optical guiding material designed within alower index regime enable greater tolerances in fabrication, whereasmaterials chosen within the higher contrast index regime give betterlight confinement. To this end the cladding material and the opticalguiding material may be chosen such that they are within the mediumindex contrast regime to provide a balance between the fabricationtolerances and light confinement.

One example of materials that are within the medium contrast indexregime is a cladding material comprising silicon dioxide and an opticalguiding material comprising silicon nitride. Silicon nitride may be anincreasingly advantageous material for edge coupler fabrication comparedto more conventional silicon designed edge couplers.

FIG. 4 is a diagram illustrating a mode field distribution of a siliconwaveguide 410, and FIG. 5 is a diagram illustrating a mode fielddistribution of a silicon nitride waveguide 510. FIG. 4 and FIG. 5 aresimulations illustrating the mode field distribution of the waveguidesobtained with the Lumerical Suite software. Due to the lower confinementof radiation in the silicon nitride waveguide 510, the mode shape has alarger cross-section than the silicon waveguide 410. The largercross-section of the silicon nitride waveguide 510 may mean that abetter matching to the mode of a light delivery means such as an opticalfibre may be more easily achieved compared to a conventional siliconwaveguide 410.

FIG. 6 is a diagram illustrating a mode field distribution of a photonicchip comprising an edge coupler according to the present disclosure.FIG. 6 illustrates the photonic chip as viewed at a first edge of thephotonic chip where the edge coupler of the chip may interface with anoptical fibre along the direction of propagation of the light along thelayers. The photonic chip comprises an upper surface 680 and a lowersurface 690. The photonic chip further comprises first optical guidinglayer 652, second optical guiding layer 654 and third optical guidinglayer 656. The optical guiding layers 652-656 may comprise any or all ofthe features and functionality of the optical guiding layers asdescribed above. The structure of the layers 652-656, as well as theother features of the composite guiding structure e.g. the separationbetween the layers, provide an overall mode distribution with a largediameter, which is configured to match that of an optical fibre. Thelarge mode may be achieved through design of the edge coupler structureas well as appropriate selection of the materials used. The photonicchip in FIG. 6 comprises layers 652-656, and an optical waveguide madeof silicon nitride, which as described above may be advantageous forcoupling light from an optical fibre to the photonic chip.

Silicon nitride demonstrates properties which may be advantageous foredge couplers. However, the restrictive nature of many conventional edgecoupler designs has meant that silicon nitride has not been a feasiblecandidate for use in edge coupler fabrication. According to an aspect ofthe present disclosure, a method of manufacturing a photonic chip isprovided, which may facilitate the use of silicon nitride.

FIG. 7 is a flow chart illustrating process steps in a method ofmanufacturing a photonic chip 700. In some examples, the method may beused to manufacture photonic chip 100 or photonic chip 200. Referring toFIG. 7 , in a first step 710, the method comprises providing claddingmaterial. In some examples, the cladding material may comprise silicondioxide. In some examples, the cladding material may be provided as partof a wafer. In such examples, the wafer may comprise silicon dioxideformed on a silicon substrate. In a second step 720, the method furthercomprises forming an edge coupler, comprising depositing a compositeguiding structure comprising a plurality of substantially parallelplanar layers of optical guiding material, wherein each layer of thecomposite guiding structure extends into the cladding material, whereineach layer is aligned at a first edge of the photonic chip and whereinthe layers overlap along a first axis which is perpendicular to a planeof the planar layers of optical guiding material In one example, theoptical guiding material may comprise silicon nitride. In a third step730, the method also comprises forming a surface of the photonic chip(e.g. upper surface) for deposition of a waveguide on the claddingmaterial, the waveguide being arranged to at least partially overlapalong the first axis with a layer of the composite guiding structure. Insome examples, the surface for deposition of the waveguide may be aplanar surface. In some examples, the surface for deposition of awaveguide may be an upper surface. In some aspects, the method 700includes forming or depositing the waveguide.

The upper surface of the photonic chip is planar above the compositeguiding structure and over an area where the waveguide will bedeposited. As such, the photonic chip is arranged for deposition of thewaveguide. The continuous planar upper surface of the photonic chip,including over the composite guiding structure, allows for addition ofthe waveguide into which light can be coupled from the composite guidingstructure. The waveguide is provided, or located, above the compositeguiding structure. As such, the photonic chip can be first provided withthe composite guiding structure. The waveguide can be added as aseparate, later and independent process. Thus, the manufacture of thecomposite guiding structure can be completed prior to addition to of thewaveguide. This separation of the manufacture of the composite guidingstructure and waveguide improves flexibility in the manufacturingprocess.

FIG. 8 is a flow chart illustrating process steps in another example ofa method of manufacturing a photonic chip 800. The method 800illustrates one way in which the steps of the method 700 may beimplemented and supplemented.

Referring to FIG. 8 , in a first step 810, the method comprisesproviding cladding material. In one example, the cladding material maycomprise silicon dioxide. In a second step 820, the method furthercomprises forming an edge coupler, comprising depositing a compositeguiding structure comprising a plurality of substantially parallelplanar layers of optical guiding material, wherein each layer of thecomposite guiding structure extends into the cladding material, whereineach layer is aligned at a first edge of the photonic chip and whereinthe layers overlap along a first axis which is perpendicular to a planeof the planar layers of optical guiding material.

Second step 820 may comprise in step 822 (a) removing a portion of thecladding material at a first end of the cladding material along a firstaxis to form a cladding material surface. In one example, the depositedcladding material may comprise a thickness of about 15 μm and step 822may comprise removing about 8 μm of cladding material from the depositedcladding material to form the cladding material surface. Step 822 mayfurther comprise in step 821 performing a stress de-compensationprocedure.

Second step 820 may also comprise in step 824, (b) fabricating a planarlayer of optical guiding material on the cladding material surface; Step824 may comprise, in step 823, depositing optical guiding material andremoving optical guiding material to fabricate the planar layer ofoptical guiding material. In one example, step 823 may comprise alithographic and etching step. In one example, the stressde-compensation procedure described in step 821, may provide a suitablesubstrate to fabricate thereon the planar layer of optical guidingmaterial, as described in step 824.

Second step 820 may further comprise instep 826, (c) depositing acladding material onto the layer of optical guiding material to form anew cladding material surface. Step 826 may comprise, in step 825,performing a stress compensation procedure.

Second step 820 may further comprise in step 828, repeating steps (b)and (c) to form a composite guiding structure comprising a plurality ofsubstantially parallel planar layers of optical guiding materialinterleaved with cladding material. In some examples, steps (c) and (d)may be repeated twice to form a composite guiding structure with threelayers of substantially parallel planar layers of optical guidingmaterial. In one example, a stress compensation procedure may be carriedout on an upper surface of the composite guiding structure. In oneexample, this may comprise depositing a further layer of claddingmaterial on the composite guiding structure.

In a third step 830, the method comprises forming a surface of thephotonic chip for deposition of a waveguide on the cladding material,the waveguide being arranged to at least partially overlap along thefirst axis with a layer of the composite guiding structure.

In a fourth step 840, the method comprises fabricating a marker elementindicating a position of the composite guiding structure. In oneexample, the marker element may be fabricated via optical lithography.In step 842 the marker element may be located on an exterior of thecladding material or may be buried in the cladding material. In oneexample, locating the marker element on an exterior surface of thecladding material may comprise depositing a CMOS-compatible materiale.g. silicon on the exterior surface of the cladding material. In oneexample, burying the marker element may comprise etching cavities in thecladding material. Following step 840, the photonic chip may comprise afull-compensated wafer structure with a planar upper surface. Such awafer may be readily suitable for fabrication of further opticalstructures thereon.

In a fifth step 850, the method comprises fabricating a waveguide on thephotonic chip. Step 850 may comprise, in step 852, aligning thewaveguide based on the marker element. Following step 850, a photonicchip is produced comprising an edge coupler that can perform modeadaptation between an optical fibre and the photonic chip.

A photonic chip according to the present disclosure can thereforeefficiently adapt the mode of an optical fibre to a waveguide that mayform part of the photonic chip. The structure of the composite guidingstructure and the arrangement of the waveguide enables this modeadaptation with minimal losses and with a small horizontal and verticalfootprint. The mode adaptation is suitable for several types of opticalfibres, thereby increasing the versatility of the photonic chip edgecoupler.

The location of the waveguide above the composite guiding structureincreases the production yield and efficiency in fabrication of thewaveguide, as testing of the composite guiding structure may be carriedout throughout its fabrication. Furthermore, the location of thewaveguide above the guiding structure also puts the waveguide underreduced stress.

The method of manufacture of a photonic chip according to the presentdisclosure, based on multi-layer solutions, is obtainable usingmaterials that are fully compliant with CMOS technology without the needfor complex fabrication processes. The method of manufacture is suitablefor manufacture using silicon nitride, which has desirable propertiesover conventional silicon, and is expected to acquire increasedimportance in future generations of transceivers with grapheneintegrated technologies.

The method of manufacture and photonic chip design enables maximumflexibility for fabrication of the waveguide, because the fabrication ofthe waveguide is completely separate from that of the composite guidestructure forming the edge coupler. The method further enables thefull-range of CMOS processes to be carried out on the chip includingannealing or top-side processing of the silicon nitride waveguide.

The chip structure allows the addition of 2D materials on top of thewaveguide cores, to implement modulation or detection functions. This isparticularly useful for silicon nitride based waveguides which requirean additional layer to achieve light-modulating action.

The chip structure also enables the realisation of a flat wafersubstrate on which other modules may be readily fabricated. Thefabrication can be adapted for either front end of line or back end ofline processes. Such processes can usefully include wafer bondingprocesses, continuous film of graphene, transition metal dichalcogenidetransfer and processing, and transfer printing processes ofsemiconductor membranes e.g. as used in hybrid photonic circuits. Suchmembranes may include indium phosphide based materials for production ofactive circuits such as lasers, amplifiers, modulators, detectors orswitches.

A photonic chip according to the present disclosure also supportspolarization diversity schemes with different types of polarizationsplitter and rotators (PSR). Such polarization diversity is notsupported by conventional photonic chips comprising edge couplers. Thecapability of supporting polarization diversity ensures the managementof arbitrary polarization states of the light in the input opticalfiber.

One type of PSR is a compact PSR based on subwavelength structures whichmay be employed with a photonic chip according to the presentdisclosure. These structures are advantageous as they comprise a smallfootprint and could be usefully employed to split and rotate thepolarization.

Another type of PSR is based on evanescent coupling with a structure inamorphous silicon. A common design for PSRs is based on adiabatictapering and symmetry breaking, which can be implemented in amorphoussilicon with a manageable footprint. The amorphous silicon structure canbe deposited on top of a photonic chip according to the presentdisclosure formed from silicon nitride so that light is transferredevanescently to the PSR. At this stage, light is split into two branches(one of which undergoes a polarization rotation) in the PSR, which iscoupled evanescently to two waveguides in the silicon nitride chipunderneath.

A further type of PSR is based on bonding a silicon-on-oxide (SOI)structure on top of a photonic chip according to the present disclosureformed from silicon nitride. This type of

PSR can be implemented by bonding a SOI chip on top of the multilayerstructure so that the light couples evanescently to a waveguide in thepolarization splitter in the SOI.

The disclosure describes a photonic chip comprising an edge coupler. Insome aspects, the disclosure may be considered as for an edge couplerfor a photonic chip or an edge coupler formed on a photonic chip. Thedisclosure describes the composite guiding structure comprising planarlayers as separate to the cladding material. In some aspects, thedisclosure may be considered as for a composite guiding structurecomprising the planar layers and the cladding material.

It should be noted that the above-mentioned examples illustrate ratherthan limit the disclosure, and that those skilled in the art will beable to design many alternative embodiments without departing from thescope of the appended claims. The word “comprising” does not exclude thepresence of elements or steps other than those listed in a claim, “a” or“an” does not exclude a plurality, and a single processor or other unitmay fulfil the functions of several units recited in the claims. Anyreference signs in the claims shall not be construed so as to limittheir scope.

1. A photonic chip comprising: cladding material; and an edge coupler,wherein the edge coupler comprises a composite guiding structurecomprising a plurality of substantially parallel planar layers ofoptical guiding material; wherein each layer of the composite guidingstructure extends into the cladding material, wherein each layer isaligned at a first edge of the photonic chip and wherein the layersoverlap along a first axis which is perpendicular to a plane of theplanar layers of optical guiding material; and wherein the photonic chipis arranged for deposition of a waveguide on the cladding material, thewaveguide being arranged to at least partially overlap along the firstaxis with a layer of the composite guiding structure.
 2. A photonic chipaccording to claim 1, wherein the photonic chip is arranged fordeposition of a waveguide on an upper surface of the cladding material.3. A photonic chip according to claim 1 wherein a width of each layer ofthe composite guiding structure decreases from a first end of the layer,which is at the first edge of the photonic chip, to a second end of thelayer opposite the first end.
 4. A photonic chip according to claim 1,wherein a length of at least one of the layers of the composite guidingstructure is different to a length of at least one other of the layersof the composite guiding structure; or, wherein a length of each layerof the composite guiding structure is different to a length of each ofthe other layers of the composite guiding structure.
 5. (canceled)
 6. Aphotonic chip according to claim 5 wherein the length of each of theplurality of layers increases with reduced distance from a surface onwhich the photonic chip is arranged for deposition of the waveguide. 7.A photonic chip according to claim 1, wherein the photonic chip isarranged for deposition of a waveguide on a surface of the claddingmaterial, wherein the waveguide is arranged to at least partiallyoverlap a layer of the composite guiding structure that is closest tothe waveguide.
 8. A photonic chip according to claim 7 wherein thephotonic chip is arranged for deposition of a waveguide on a surface ofthe cladding material, wherein the waveguide is arranged to at leastpartially overlap only the layer of the composite guiding structure thatis closest to the waveguide.
 9. A photonic chip according to claim 1wherein the composite guiding structure comprises three substantiallyparallel planar layers of optical guiding material.
 10. A photonic chipaccording to claim 1 wherein the composite guiding structure comprisesan upper layer, a middle layer, and a lower layer, and wherein a lengthof the middle layer is greater than twice a length of the lower layer.11. A photonic chip according to claim 10 wherein the composite guidingstructure comprises an upper layer, a middle layer, and a lower layer,and wherein a length of the upper layer is greater than twice a lengthof the middle layer.
 12. (canceled)
 13. (canceled)
 14. (canceled)
 15. Aphotonic chip according to claim 1 wherein a separation along the firstaxis between the each of the plurality of layers in the compositeguiding structure is equal.
 16. A photonic chip according to claim 1wherein a separation along the first axis between the composite guidingstructure and a surface of the cladding material for deposition of awaveguide thereon, is greater than the separation between each of theplurality of the layers.
 17. A photonic chip according to claim 1,further comprising a marker element, the marker element indicating aposition of the edge coupler.
 18. (canceled)
 19. A photonic chipaccording to claim 1 wherein the waveguide comprises a first end that atleast partially overlaps with a layer of the composite guiding structurein a direction of propagation of light along a layer of the plurality oflayers, and wherein a width of the waveguide increases away from thefirst end.
 20. A method of manufacturing a photonic chip, the methodcomprising: providing cladding material; forming an edge coupler,comprising depositing a composite guiding structure comprising aplurality of substantially parallel planar layers of optical guidingmaterial; wherein each layer of the composite guiding structure extendsinto the cladding material, wherein each layer is aligned at a firstedge of the photonic chip and wherein the layers overlap along a firstaxis which is perpendicular to a plane of the planar layers of opticalguiding material; and forming a surface of the photonic chip fordeposition of a waveguide on the cladding material, the waveguide beingarranged to at least partially overlap along the first axis with a layerof the composite guiding structure.
 21. A method according to claim 20wherein forming the edge coupler comprises: (a) removing a portion ofthe cladding material at a first end of the cladding material along afirst axis to form a cladding material surface; (b) fabricating a planarlayer of optical guiding material on the cladding material surface; (c)depositing a cladding material onto the layer of optical guidingmaterial to form a new cladding material surface; and (d)repeating steps(b) and (c) to form a composite guiding structure comprising a pluralityof substantially parallel planar layers of optical guiding materialinterleaved with cladding material.
 22. A method according to claim 21wherein step (a) comprises performing a stress de-compensationprocedure, and wherein step (c) comprises performing a stresscompensation procedure.
 23. A method according to claim 21 or 22 whereinstep (b) comprises depositing optical guiding material and removingoptical guiding material to fabricate the planar layer of opticalguiding material.
 24. A method according to claim 20 further comprisingfabricating a marker element indicating a position of the compositeguiding structure.
 25. (canceled)
 26. A method according to claim 20further comprising fabricating a waveguide on the photonic chip; andwherein fabricating the waveguide comprises aligning the waveguide basedon the marker element.
 27. (canceled)